This exhibit is a Mullard SD4, it has gold external metalising and sadly our exhibit has lost its top cap. The SD4 is a diode tetrode and was designed to be used as the detector and first audio stage in broadcast receivers. It was introduced in 1933 just as the world moved to the superhet for receiver design. The single diode is for signal rectification and without a second diode for AGC rectification the SD4 was limited to use in TRF receivers.
For universal sets to be used in areas when only DC mains existed the valve was fitted with a 180 mA heater and called the SD20.
The SD4 is an unusual valve and was made for only a short time and examples are not commonly found. The literature indicates that the successor to the SD4 was the TDD4. In fact Mullard introduced both valves in 1933. The data book for that year shows that the TDD4 sold for 15/6 whereas the SD4 sold for 20/-.
Looking into the dome of the envelope the inner structure becomes visible. The screen grid is a solid sided cage with close wound wires on the open faces the classic screen and far removed from the later helical screen grid. The anodes are thin vertical strips of bright metal. From the pattern of adhesive it is believed that the top cap would have been of the large Bakelite form with a screw top connection.
Mullard say that the optimum screen grid voltage would be 50 Volts and the optimum anode resistor would be 25,000 Ohms.
The balloon envelope is 50 mm in diameter, and excluding the B7 base pins is 106 mm tall.
References: Datasheet, 4024 & 1043. Type SD4 was first introduced in 1933.
Absolute Maximum Operating Conditions¶
Thanks to Andy Cowley for supplying the above PDF datasheet.
Updated December11, 2015.